To increase the productivity of the steel-making process, measures are required which on the one hand allow the inclusion in the bath of a maximum of iron-bearing material, such as scrap iron and/or rich ores, and on the other hand assures an improved quality by eliminating from the produced steel, as far as possible, phosphorus and sulphur contained in the charge.
It is known to monitor the evolution of the slag during the refining cycle and to adjust either the oxygen-lance blow rate or the height of the lance head above the level of the bath. In fact, the division of the blown oxygen between the slag and the molten metal can be, to a certain extent, adjusted by altering--with a steady oxygen-discharge rate and a particular lance-head configuration--the distance between the head of the lance and the bath level.
An increased height of the lance head results in the preferential oxidation of the slag which then assumes a frothy consistency, thus enhancing the dephosphorization and the desulphurization. On the other hand, a reduced height of the lance head leads to an accelerated decarburization and to an increased release of heat, principally at the oxygen-jet impact point, the heat of which can assist in the melting of the solid matter contained in the bath.
However, despite the development of expensive special lances with the aim to increase the extent of post-combustion or afterburning of CO at the bath surface, all imaginable efforts to increase the temperature at the bath surface with the intention to melt the remainder of the scrap are thwarted by the presence on the bath surface of a layer of thick and frothy slag which develops during the refining by top-blown oxygen and which acts as a thermal insulator because of its frothy consistency.
With these thoughts in mind, there has been developed a refining process which allows for an increase in the traditional levels of the scrap-iron additions and, at the same time, avoids the above-described disadvantages. That process, described in commonly owned Luxembourg patent application No. 81,207, provides for the refining of the bath by top-blown oxygen and, in the first place, promotes in the immediate proximity of the bath surface a post-combustion of carbon monoxide released during the decarburization by spreading the oxygen over that surface and, secondly, continually controls the thickness and the consistency of the slag by acting on the disequilibrium between the slag and the bath through the bottom-blown injection of an essentially inert gas, thereby enabling passage of the requisite amount of oxygen through the slag layer.
However, a refining process during which at all times the interface between the metal and the slag is continually swept by the addition of agitation gas, so that the slag remains always deoxidized and as a result cannot assume a frothy consistency, does not facilitate by itself the multiplicity of reactions which occur in the bath and especially in the slag.
In fact, the slag must necessarily have an adjustable level of reactivity as well as an essentially fluid consistency, equally adjustable, to allow the refining under conditions known to be favorable both from the point of view of afterburning of CO and from the point of view of the dephosphorization and desulphurization of the bath. At the same time, it is necessary to monitor the instantaneous level of decarburization of the bath as a function of the injected oxygen.